Three farms where five badgers were killed in a controversial trial were accidentally revealed by the Welsh Government in "error".
The names and locations of the farms were revealed by mistake in a freedom of information request response.
It led to a group campaigning against the trial naming the farms in a Facebook post, asking supporters to "monitor activity" there.
The Welsh Government said it was now "urgently reviewing procedures".
The trial - on farms with long-standing TB issues - involved trapping badgers, testing them and then if they were infected, killing them with an injection.
The post by the group, which is not being identified by the BBC to protect the safety of the farmers, said five badgers had been killed at the farms at a cost of more than £380,000.
It added: "Documents released as a result of a Freedom of Information request show the detailed work plan drawn up by the Animal and Plant Health Agency for each of these farms, including their names.
"Operations may have already started at these farms this year. We are asking people to monitor activity at these farms, looking for unusual activity that could indicate killing has restarted and let us know."
The group denied it was "irresponsible" for sharing the information.
A Welsh Government spokeswoman said: "Due to an administrative error, a document sent in response to a freedom of information request was released without removing an element of personal data.
"We are urgently reviewing our procedures in light of this error."
An Information Commissioners's Officer spokesperson said: "We are aware of an incident relating to the Welsh Government and will be making enquiries."
Analysis by Steffan Messenger, BBC Wales environment correspondent
The Welsh Government's trial to trap, test and kill infected badgers on farms with long-standing TB issues is very different to the approach in England, where there has been a more widespread cull.
But for animal rights activists it is no less controversial.
The fact that one campaign group's now got its hands on the details of the farms involved - which was meant to be kept top secret - has troubled farming leaders.
Though some said they'd rather not do interviews today, in case it draws more attention to the farms.